Bandpass filters heretofore used in the radio-frequency band ranging from the VHF band nearly to microwaves are comb line filters and interdigital line type filters, in which resonant lines are formed within an envelope made from a conductor such as a metal. The inside of the envelope is permeated with air or kept in vacuum. This air space or vacuum constitutes a medium through which electromagnetic waves propagate between the resonant lines.
A conventional three-stage interdigital type filter of this kind is shown in FIGS. 6 and 7. This filter has an envelope 1 and its cover 2, both of which are made of a conductive metal. Two metal rods acting as exciter lines 3 and 4 are disposed on opposite sides within the envelope 1. Three metal rods serving as resonant lines 5, 6, 7 are substantially regularly spaced from each other between the exciter lines 3 and 4. The envelope 1 is provided with holes 1a and 1b on its one side, and the exciter lines 3 and 4 protrude outwardly through the holes 1a and 1b, respectively. These protruding portions form an input terminal 3a and an output terminal 4a. The exciter lines 3 and 4 are held to the inner wall of the envelope 1 at their rear ends 3b and 4b that are short-circuited surfaces. The three resonant lines 5, 6, 7 have open surfaces 5a, 6a, 7a and short-circuited surfaces 5b, 6b, 7b, respectively, at their opposite ends Any neighboring two of the open surfaces 5a-7a are on opposite sides. Also, any neighboring two of the short-circuited surfaces 5b-7b are on opposite sides. The resonant lines 5-7 are held to the inner wall of the envelope 1 at their short-circuited surfaces 5b-7b. Although the inside of the envelope 1 may be kept in vacuum, it is permeated with air in the illustrated example. Accordingly, space is left between the open surfaces 5a-7a of the resonant lines 5-7 and the opposite inner wall of the envelope 1. The resonant lines 3 and 4 excite the resonant lines 5-7 and perform transformation of impedance. Since the resonant lines 5, 6, 7 exhibit bandpass characteristics, the interdigital line type filter functions as a bandpass filter.
Another conventional filter is shown in FIGS. 8 and 9. This filter is similar to the above-described filter except that the space inside the envelope is filled with a dielectric substance having a high dielectric constant, such as ceramics, and except for the respects described below. The dielectric substance forms a rectangular block 8 having opposed wall surfaces 8a and 8b. Two holes 9c and 10c extend in a parallel relation at a suitable interval through the block 8 between the wall surfaces 8a and 8b to form exciter lines. Three parallel holes 11c, 12c, 13c extend through the block 8 between the holes 9c and 10c in a substantially regularly spaced relation from one another. Every other holes 11c and 13c reach the side wall surface 8a, while the intervening hole 12c reaches the opposite side wall surface 8b. The inner walls of the holes 9c, 10c, 11c, 12c13c and the outer surface of the dielectric block 8 are coated with a metal by electroless plating, or they are coated with conductive paste or the like by baking, whereby electrode films are formed on them. A grounding electrode 14 is formed on the outer surface. Exciter lines 9 and 10 are formed in the holes 9c and 10c, respectively. Resonant lines 11, 12, 13 are formed in the holes 11c, 12c, 13c, respectively. The short-circuited ends 9b and 10b of the exciter lines 9 and 10 are connected to the grounding electrode 14 on the side wall surface 8b. Those portions of the grounding electrode 14 which are in the vicinities of the open ends 9a and 10a have been removed. Input and output terminals are brought out from the open ends 9a and 10a, respectively. The short-circuited ends 11b, 12b, 13b of the resonant lines 11, 12, 13 are connected to the grounding electrode 14 in the same way as the foregoing. The dielectric substance 8 occupies the space between the open ends 11a, 12a, 13a and the opposite grounding electrode 14.
Since the wavelengths of electromagnetic waves shorten in a dielectric substance having a high dielectric constant, the lines 9-13 can be made much shorter than the resonant wavelength. This filter is manufactured in much smaller size than the filter already described in connection with FIGS. 6 and 7, but its electrical actions including the creation of the bandpass characteristics are similar to those of the first-mentioned filter.
In the conventional filter described first, the inside of the envelope 1 is either permeated with air or kept in vacuum. Since electromagnetic waves propagate through the medium, i.e., air or vacuum, having a specific dielectric constant of 1, the medium does not allow the waves to shorten their wavelengths. For this reason, the lines 3-7 are long. Further, the envelope 1 and other components are large in size. Hence, the filter is large in size and heavy in weight.
In the conventional filter already described in connection with FIGS. 8 and 9, electromagnetic waves propagate through a dielectric substance having a high dielectric constant and so the wavelengths of the waves shorten. This allows the lines 9-13 to be manufactured in much shorter lengths. In this way, this filter has solved the problems with the first-mentioned conventional filter.
However, these two conventional filters still suffer from the same problem that the performance of the actually fabricated product deviates considerably from the designed characteristics.
This problem is further discussed below. Referring again to FIGS. 8 and 9, the lines 9-13 are coupled by electromagnetic field. The couplings planned at the stage of the designing of the filter are simply the couplings between neighboring lines, e.g., between the exciter line 9 and the resonant line 11 and between the resonant lines 11 and 12. In reality, however, when the filter functions actually, couplings occur between next lines but one, i.e., between the lines 9 and 12, between the lines 11 and 13, between the lines 12 and 10. If these couplings between next lines but one are also taken into account at the stage of design, the equation for design will become so complex that its analysis is almost impossible. Therefore, such couplings have not been included in the calculation. It is thought that the deviation of the performance of the actual product from the designed performance is due to the effects of such couplings. These undesired couplings are explained in the manner described below. Let us take the resonant line 11 shown in FIG. 9 by way of example. No electrode body exists between the open end 11a and the opposite grounding electrode 14, and therefore electromagnetic waves easily propagate. The lines 9 and 12 are coupled together primarily through this gap. For the same reason, the lines 11 and 13 are coupled together, and the lines 12 and 10 are coupled together
Substantially the same situation applies to the first-mentioned conventional filter. That is, next lines but one are coupled together through the gaps between the open surfaces 5a, 6a, 7a of the resonant lines 5, 6, 7 and the opposite metal envelope 1. These couplings result in the deviation.